XRD and NMR investigation of Ti-compound formation in solution-doping of sodium aluminum hydrides: solubility of Ti in NaAlH4 crystals grown in THF

Sodium aluminum hydrides have gained attention due to their high hydrogen weight percent (5.5% ideal) compared to interstitial hydrides, and as a model for hydrides with even higher hydrogen weight fraction. The purpose of this paper is to investigate the Ti-compounds that are formed under solution-doping techniques, such as wet doping in solvents such as tetrahydrofuran (THF). Compound formation in Ti-doped sodium aluminum hydrides is investigated using X-ray diffraction (XRD) and magic angle spinning (MAS) nuclear magnetic resonance (NMR). We present lattice parameter measurements of crushed single crystals, which were exposed to Ti during growth. Rietveld refinements indicate no lattice parameter change and thus no solubility for Ti in NaAlH4 by this method of exposure. In addition, X-ray diffraction data indicate that no Ti substitutes in NaH, the final decomposition product for the alanate. Reaction products of completely reacted (33.3 at.%-doped) samples that were solvent-mixed or mechanically milled are investigated. Formation of TiAl3 is observed in mechanically milled materials, but not solution mixed samples, where bonding to THF likely stabilizes Ti-based nano-clusters. The Ti in these clusters is activated by mechanical milling.